In the field of semiconductor metrology, a metrology tool may comprise an illumination system which illuminates a target, a collection system which captures relevant information provided by the illumination system's interaction (or lack thereof) with a target, device or feature, and a processing system which analyzes the information collected using one or more algorithms. Metrology tools can be used to measure structural and material characteristics (e.g., material composition, dimensional characteristics of structures and films such as film thickness and/or critical dimensions of structures, overlay, etc.) associated with various semiconductor fabrication processes. These measurements are used to facilitate process controls and/or yield efficiencies in the manufacture of semiconductor dies. Metrology tools may include one or more hardware configurations which may be used in conjunction with certain embodiments of this invention to, e.g., measure the various aforementioned semiconductor structural and material characteristics. Examples of such hardware configurations include the following: a spectroscopic ellipsometer (SE), a SE with multiple angles of illumination, a SE measuring Mueller matrix elements (e.g., using rotating compensator(s)), a single-wavelength ellipsometers, a beam profile ellipsometer (angle-resolved ellipsometer), a beam profile reflectometer (angle-resolved reflectometer), a broadband reflective spectrometer (spectroscopic reflectometer), a single-wavelength reflectometer, an angle-resolved reflectometer, any imaging system, a pupil imaging system, a spectral imaging system, a scatterometer (e.g., speckle analyzer) etc.
The hardware configurations can be separated into discrete operational systems. On the other hand, one or more hardware configurations can be combined into a single tool. One example of such a combination of multiple hardware configurations into a single tool is provided by U.S. Pat. No. 7,933,026 (including e.g., a broadband SE, a SE with rotating compensator, a beam profile ellipsometer, a beam profile reflectometer, a broadband reflective spectrometer, and a deep ultra-violet reflective spectrometer) which is incorporated herein by reference in its entirety. In addition, there are typically numerous optical elements in such systems, including certain lenses, collimators, mirrors, quarter-wave plates, polarizers, detectors, cameras, apertures, and/or light sources. The wavelengths for optical systems can vary from about 120 nm to 3 microns. For non-ellipsometer systems, signals collected can be polarization-resolved or unpolarized. Multiple metrology heads may be integrated on the same tool, however, in many cases, multiple metrology tools are used for measurements on a single or multiple metrology targets, as described e.g., in U.S. Pat. No. 7,478,019, which is incorporated herein by reference in its entirety.
The illumination system of the certain hardware configurations includes one or more light sources. The light source may generate light having only one wavelength (i.e., monochromatic light), light having a number of discrete wavelengths (i.e., polychromatic light), light having multiple wavelengths (i.e., broadband light) and/or light that sweeps through wavelengths, either continuously or hopping between wavelengths (i.e., tunable sources or swept source). Examples of suitable light sources are: a white light source, an ultraviolet (UV) laser, an arc lamp or an electrode-less lamp, a laser sustained plasma (LSP) source, a supercontinuum source (such as a broadband laser source), or shorter-wavelength sources such as x-ray sources, extreme UV sources, or some combination thereof. The light source may also be configured to provide light having sufficient brightness, which in some cases may be a brightness greater than about 1 W/(nm cm2 Sr). The metrology system may also include a fast feedback to the light source for stabilizing its power and wavelength. Output of the light source can be delivered via free-space propagation, or in some cases delivered via optical fiber or light guide of any type.
The metrology targets may possess various spatial characteristics and are typically constructed of one or more cells which may include features in one or more layers which may have been printed in one or more lithographically distinct exposures. The targets or the cells may possess various symmetries such as two fold or four fold rotation symmetry, reflection symmetry, as described e.g., in U.S. Pat. No. 6,985,618, which is incorporated herein by reference in its entirety. Different cells or combinations of cells may belong to distinct layers or exposure steps. The individual cells may comprise either isolated non-periodic features or alternately they may be constructed from one, two or three dimensional periodic structures or combinations of non-periodic and periodic structures as e.g., in U.S. Patent Publication No. 2013/042089, which is incorporated herein by reference in its entirety. The periodic structures may be non-segmented or they may be constructed from finely segmented features which may be at or close to the minimum design rule of the lithographic process used to print them. The metrology targets may also be collocated or in close proximity with dummification structures in the same layer or in a layer above, below or in between the layers of the metrology structures. Targets can include multiple layers (or films) whose thicknesses can be measured by the metrology tool. Targets can include target designs placed (or already existing) on the semiconductor wafer for use, e.g., with alignment and/or overlay registration operations. Certain targets can be located at various places on the semiconductor wafer. For example, targets can be located within the scribe lines (e.g., between dies) and/or located in the die itself. Multiple targets may be measured (at the same time or at differing times) by the same or multiple metrology tools as described e.g., in U.S. Pat. No. 7,478,019, which is incorporated herein by reference in its entirety. The data from such measurements may be combined. Data from the metrology tool is used in the semiconductor manufacturing process for example to feed-forward, feed-backward and/or feed-sideways corrections to the process (e.g., lithography, etch), see e.g., U.S. Pat. No. 8,930,156, which is incorporated herein by reference in its entirety, disclosing feed forward methods for reusing metrology target cells; and therefore, might yield a complete process control solution. The metrology tools are designed to make many different types of measurements related to semiconductor manufacturing, for example measure characteristics of one or more targets, such as critical dimensions, overlay, sidewall angles, film thicknesses, process-related parameters (e.g., focus and/or dose). The targets can include certain regions of interest that are periodic in nature, such as for example gratings in a memory die.
As semiconductor device pattern dimensions continue to shrink, smaller metrology targets are often required. Furthermore, the measurement accuracy and matching to actual device characteristics increase the need for device-like targets as well as in-die and even on-device measurements. Various metrology implementations have been proposed to achieve that goal. For example, focused beam ellipsometry based on primarily reflective optics is described e.g., in U.S. Pat. No. 5,608,526, which is incorporated herein by reference in its entirety. Apodizers can be used to mitigate the effects of optical diffraction causing the spread of the illumination spot beyond the size defined by geometric optics, as described e.g., in U.S. Pat. No. 5,859,424, which is incorporated herein by reference in its entirety. The use of high-numerical-aperture tools with simultaneous multiple angle-of-incidence illumination is another way to achieve small-target capability, as described e.g., in U.S. Pat. No. 6,429,943 which is incorporated herein by reference in its entirety. Other measurement examples may include measuring the composition of one or more layers of the semiconductor stack, measuring certain defects on (or within) the wafer, and measuring the amount of photolithographic radiation exposed to the wafer. In some cases, metrology tool and algorithm may be configured for measuring non-periodic targets, as described e.g., in U.S. patent application Ser. No. 14/294540 and in U.S. Patent Publication No. 2014/0222380, which are incorporated herein by reference in their entirety.
Measurement of parameters of interest usually involves a number of algorithms, carried out by corresponding analysis units in the respective metrology tools. For example, optical interaction of the incident beam with the sample is modeled using EM (electro-magnetic) solver and uses such algorithms as RCWA (Rigorous Coupled Wave Analysis), FEM (finite element method), method of moments, surface integral method, volume integral method, FDTD (Finite Difference Time Domain), and others. The target of interest is usually modeled (parameterized) using a geometric engine, or in some cases, process modeling engine or a combination of both. The use of process modeling is described e.g., in U.S. Patent Publication No. 2014/0172394, which is incorporated herein by reference in its entirety. A geometric engine is implemented, for example, in AcuShape software product of KLA-Tencor.
Collected data can be analyzed by a number of data fitting and optimization techniques and technologies including libraries, Fast-reduced-order models; regression; machine-learning algorithms such as neural networks, support-vector machines (SVM); dimensionality-reduction algorithms such as, e.g., PCA (principal component analysis), ICA (independent component analysis), LLE (local-linear embedding); sparse representation such as Fourier or wavelet transform; Kalman filter; algorithms to promote matching from same or different tool types, and others. Collected data can also be analyzed by algorithms that do not include modeling, optimization and/or fitting modeling as described e.g., in U.S. Patent Publication No. 2014/0257734, which is incorporated herein by reference in its entirety. Computational algorithms are usually optimized for metrology applications with one or more approaches being used such as design and implementation of computational hardware, parallelization, distribution of computation, load-balancing, multi-service support, dynamic load optimization, etc. Different implementations of algorithms can be done in firmware, software, FPGA (Field Programmable Gate Array), programmable optics components, etc. The data analysis and fitting steps usually pursue one or more of the following goals: Measurement of CD, SWA, shape, stress, composition, films, bandgap, electrical properties, focus/dose, overlay, generating process parameters (e.g., resist state, partial pressure, temperature, focusing model), and/or any combination thereof; modeling and/or design of metrology systems; and modeling, design, and/or optimization of metrology targets.